Introduction to skylights

500cm radius skylight of 69 bulb lights, 300cm width and 7 rays each

The effect of the sky light is more apparent during a cloudy day where objects are illuminated by the diffuse light from the sky above. Because the sky surrounds the objects all over, the lighting effect is very diffuse and the shadows are also very diffused and almost non-existent. Instead of receiving harsh shadows, receded parts of objects are still illuminated but receive much less light than exposed parts, thus, producing nice gradations of lights illuminating different parts of the objects.

Same skylight as above. A hemisphere model with a gradient material at 50% transparency is fitted right under the skylight to serve as a skydome and to color filter the skylight lights. the Skylight light intensity have been reduced to give prominence to the added sun light which have an orangeish tint.

On a sunny day, outside objects are also illuminated directly by the sun. Since the interaction of the oxygen and nitrogen molecules and the incident rays coming from the sun scatters much more the blue spectra of the sun light into the atmosphere, the sky itself appears blue because it is filled with those scattered blue rays. And since the blue spectra of the sun light is scattered into the atmosphere, what is left from the sun light that reaches the objects directly appears yellow because it is depleted from its blue spectra through this scattering. Sunny scenes are often reproduces in paintings with exaggerated bluish shadows and yellowish bright areas.

During sunrise or sunset, the thickness of the atmosphere that the rays have to travel is so much more thick that the scattering not only affect the blue spectra but also the green and the yellow spectra which produces a sun with a very pronounced red tint.

Skylight rigs are devices designed to simulate this scattered light that comes from the sky all around the objects in an outside scene. Skylights are not perfect because they do not allow the re-scattering of lights that bounces off other objects or other parts of the same objects.

Skylight are compromises. They do not simulate the whole phenomena but they take much less computing resources. They can also be implemented in any renderer that can support several light sources simultaneously. Still, because they use large arrays of lights, they take proportionately more time to render. For instance, if a scene illuminated with the standard 3 light setup takes 1 minute to render, the same scene illuminated with 1 main light and a skylight of 32 lights will take about 11 minutes because there are 11 times the number of lights. If in addition, each light emits 2 rays to soften the shadows and the star pattern in shadows, the scene will take about 22 minutes to render. Now if the same scene is rendered with oversampling and antialiasing on, a render time of more than 6 hours is to be expected.

For more on skylights, continue to :

• Different approaches to skylight. A look at different way of organizing lights in a skylight.
• Skylight library is a catalog of the different skylights available for download here.
• Geodesic dome types covers the basics of geodesic domes and how they can be used as a basis for skylights.
• Skylight setup shows how to setup a basic skylight in a choreography.
• Customizing a Skylight examine different alternatives for light types, width and number of rays in a skylight.
• Technical analysis offers a detailed analysis of the effect of different light types, width and ray casts on the render quality.
• Advanced techniques for Skylights for a review of some nice tricks that can be used with skylights.